The long-term objective of the studies outlined in this proposal is to investigate whether different pain modalities are conveyed to the brain via distinct neuronal circuits and to elucidate the mechanisms by which these signals are regulated under normal conditions and in the setting of injury-induced persistent pain. There is evidence for specialization of primary afferent 'pain' fibers for the detection of different noxious stimulus modalities but whether modality-specific afferents engage separate spinal and supraspinal cord circuits, is unclear. Experiments proposed in this application will investigate the neuronal circuis engaged by the TRPV1- expressing population of primary afferents, which is critical for the detection of thermal (heat) stimuli, and the MrgprD-expressing primary afferents, which have been implicated in the response to mechanical stimuli. In related experiments we will investigate the neuronal circuits engaged by descending brainstem serotonergic neurons, which modulate the processing of ascending 'pain' signals at the level of the spinal cord. These tract- tracing studies will be performed using an innovative approach in which anterograde transneuronal transfer of a virus can be triggered in defined subsets of neurons, using expression of the Cre recombinase to induce the virus. This novel tracing method will allow for the sensitive visualization of neuronal circuits engaged by distinct 'nociresponsive' cells and the serotonergic regulatory neurons of the brainstem. Results from these studies will contribute to the development of novel/more effective pain therapies targeted at the neuronal populations and circuits responsible for transmitting distinct modalities of pain. PUBLIC HEALTH RELEVANCE: In our proposal we will use a novel tracing method to uncover the neuronal pathways engaged by the two main classes of primary afferent pain fibers as well as by neurons in the caudal brainstem that modulate spinal cord pain messages. These studies are essential for understanding how organisms interpret different 'types' of pain and may reveal novel cellular targets for the development of novel pain treatments.